Phosphate optical glass containing P2O5 as its main component is valuable optical glass which is used as a material for various optical elements because it exhibits low dispersion as its characteristics. On the other hand, since P2O5 is soluble in water, phosphate optical glass has a relatively low weather resistance, which has been a problem. Therefore, attempts to enhance the weather resistance of phosphate optical glass have been made. For example, JP-A No. 63-11544 and U.S. Pat. No. 7,598,189 disclose phosphate optical glass with improved weather resistance prepared by determining the ratios of composition to be prescribed ranges.
In recent years, an increasing number of equipments, such as portable equipments and onboard equipments, have been used in a severe environment. Therefore, phosphate optical glass has been required to have higher weather resistance. However, it is still difficult to say that phosphate optical glass disclosed in JP-A No. 63-11544 and U.S. Pat. No. 7,598,189 has sufficient weather resistance, and it has been a problem that the optical glass hardly meet the above demand.